All biological energy ultimately comes from solar or geothermal energy harnessed by autotrophic organisms Chemoautotrophs Photoautotrophs Photosynthesis occurs in 5 of the 9 phylogenetic divisions of eubacteria

Bacteriorhodopsin, the simplest form of phototrophy

All trans retinal 13-cis retinal

CO 2 + 2H 2 S ---> (CH 2 O) + 2S + H 2 O Photosynthetic green sulfur bacteria Hypothesis CO 2 + 2H 2 A ---> (CH 2 O) + 2A + H 2 A h h A = oxygen in cyanobacteria and plants and anoxic sulfur in green and purple sulfur bacteria

CO 2 + 2H 2 A ---> (CH 2 O) + 2A + H 2 A 2 half reactions 2H 2 A ---> 2A + 4[H] 4[H] + CO 2 ---> (CH 2 O) + H 2 O Light reactions h Dark reactions

Electron micrograph of a section through the purple photosynthetic bacterium Rhodobacter sphaeroides.

Amount needed to make ATP = kJ E = h = hc/ The nature of light h = x Js c = x 10 8 ms -1

Amount of light absorbed is a funtion of the physical properties of the absorbing medium A = log I 0 /I =  cl  = molar extinction coefficient M -1 cm -1 ) c = concentration (M) l = pathlength (cm)  for chlorophylls are among the highest for organic molecules ≈ 10 5 M -1 cm -1

Absorbed energy can be dissipated in several ways Internal conversion: very fast < s Electronic energy is converted to kinetic energy Fluorescence: very fast ≈ 10 8 s Absorbed energy is re-emitted generally at a lower energy/longer wavelength

Absorbed energy can be dissipated in several ways Excition transfer: slow Energy is passed from molecule to molecule Photoxidation: slow Energy is transferred to a photosynthetic reaction system

The amount of O 2 evolved by Chlorella algae versus the intensity of light flashes. 300 chlorophyll/ Reaction center

Since there is an excess of chlorophyll it is unlikely that all of them function as reaction centers Most are act as antennae to harvest light from a variety of wavelengths and transfer it to a single reaction center

What is the nature of the antennae

Take the example of purple non-sulfur bacteria  -proteobacteria

Light harvesting complex 2 Green = bacteriochlorophyll a Yellow = lycopene

Light harvesting complex 1 surrounds the reaction center Its chlorophyll is slightly lower in energy to facilitate exciton transfer Ultimately all the photons harvested make their way to the reaction center

Cyanobacteria PE = phycoerythrin PC = phycocyanin AP = allophyocyanin

Light harvesting complexes in plants are much more complex and have a wide array of pigment molecules  -Carotene Phycocyanobilin

LH2 from pea Green = chlorophyll a Red = chlorophyll b Yellow = lutein

Alpha proteobacteria

(Chl) exciton ---> (Chl)* 2 (Chl)* 2 + Pheo ---> (Chl) Pheo - 2 Pheo - + 2H + + Q B ---> 2Pheo + Q B H 2 ∆E’º = +0.95V !!!!

Coenzyme Q Ubiquinone CoQ Q Redox loops pumps out four protons!

Photosynthetic electron-transport system of purple photosynthetic bacteria. Related to complex III

Oxidation of sulfur Electrons taken from reaction center to reduce NAD + are replaced by the oxidation of H 2 S to S 0 and SO 4 2-

Plants and cyanobacteria FeS type Pheo type

Ubiquinone Plastoquinone

Net reaction 2H 2 O + 2NADP photons ---> O 2 + 2NADPH + 2H + 4P H + + 2PQ B + 4photons ---> 4P PQ B H 2

Oxygen evolving Complex

In cyanobacteria plastocyanin is be replaced by a small cytochrome c like protein Cyt c 6 can perform both roles in this bacterium

Photosystem I is related to bacterial FeS type photosystem

2Fd red + 2H + + NADP + ---> 2Fd ox + NADPH + H + During Cu deficiency plastocyanin can be replaced with a cytochrome c like molecule

About 3ATP are made per O 2 produced 2H 2 O + 8 photons + 2NADP + + 3ADP + 3P i ---> O 2 + 3ATP + 2NADPH

Cyclic pathway does not generate NADPH

Photosystem I and II are spatially separated to prevent exciton transfer and loss of proton gradient Photosystem I in unstacked stroma lamellae Photosystem II in closely stacked grana

The Calvin cycle. 3CO > GAP 9 ATP and 6 NADPH

3C5 3C1 6C3 1C3 C6 C3+C3 C3+C4 C6+C3 C5 C4 C7+C3 C7 C5

C 3 + C 3 ---> C 6 C 3 + C 6 ---> C 4 + C 5 C 3 + C 4 ---> C 7 C 3 + C 7 ---> C 5 + C 5 Overal reaction = 5C 3 ---> 3C 5 1 GAP molecule is made from 3CO 2 3CO 2 + 9ATP + 6NADPH ---> GAP + 9ADP + 8P i + 6NADP+ GAP is converted to glucose by gluconeogenesis aldolase transketolase aldolase transketolase 3C 5 + 3C 1 ---> 6C 3

Photorespiration Dissipates ATP and NADPH What is the purpose? To protect from photo oxidation in the absence of CO 2 ?

On a hot bright day CO 2 may be depleted and O 2 may accumulate Under these conditions photorespiration may take over This may prevent the photooxidation of reaction centers By decreasing photorespiration plants save water because they do not have to have their pores open to acquire CO 2

C 4 plants (such as grasses) reduce photorespiration by physically separating CO 2 and O 2 acquisition from rubisco These plants assimilate CO 2 in mesophyll cells as malate and transporting this to the site of rubisco in bundle-sheath cells C 4 plants outgrow C 3 plants on hot days It uses more ATP to make sugars

Another type of plants called CAM plants use a variant of the C 4 cycle In this case CO 2 acquisition is temporally separated from rubisco At night when the air is cool and moist CAM plants open their pores and let CO 2 in. The CO 2 is incorporated into malate and stored in the vacuole. During the day the CO 2 is released from malate and there is a steady supply of CO 2 to prevent photorespiration.

Control of the Calvin Cycle Phosphoribulokinase Rubisco Phosphoglycerate kinase/GAPDH Fructose bisphosphatase Sedoheptulose bisphosphatase

Regulation of enzymes by light Phosphoribulokinase Glyceraldehyde-3-phosphate dehydrogenase Fructose bisphosphatase Sedoheptulose bisphosphatase

What about Rubisco? Responds to light dependent factors pH of stroma increases by 1 unit when photosynthesis is on. Rubisco has a pH optimum at pH 8.0 Rubisco is activated by Mg 2+, light induced influx of H + into lumen is accompanied by Mg 2+ efflux into stroma